1. Field of the Invention
This invention relates to systems and methods for controlling the oxygen concentration, and the carbon dioxide concentration, and for monitoring the temperature, in shipping and storage containers for fresh fruits and vegetables and other perishables.
2. Description of the Prior Art
Methods and apparatus using gas atmospheres to preserve the quality of refrigerated perishable fruits and vegetables are known, but have not been widely used. Some systems try to make transport containers sufficiently gastight to allow respiration of perishables to lower the oxygen level, in the containers or to maintain an injected gas atmosphere in the containers. In such systems, the respiration rate of the perishables is used to lower or maintain a desired oxygen level. Carbon dioxide concentration in such containers has been passively reduced using hydrated lime. Undesirable gases, such as ethylene, were removed by reaction with other substances to form new, harmless substances. Other systems used nitrogen in large quantities to flush excess oxygen and carbon dioxide from containers for perishable fruits and vegetables.
These systems failed to detect instantaneous concentrations of gases such as oxygen and carbon dioxide, and failed to take account of, and correct for fortuitous events such as unpredictably high or low respiration rates in perishables, transportation or storage equipment that was less gastight than desired, or poor circulation of the atmosphere surrounding the perishables.
Some systems and methods sought to maintain desired levels of oxygen and carbon dioxide in shipping containers, but these systems required elaborate, costly modifications to the container at the time of its manufacture. Such systems required specialized piping and methods for replenishing the gaseous atmosphere within the containers. Such systems obtained replenishing gases by vaporizing onboard, stored gases, or utilized sieves, membranes or other means to separate oxygen from air.
Some of these systems used removable controllers. However, because of the required piping, or because of the resulting need for enlarged electrical storage means in the containers, such systems were costly. These systems did not permit exposure of the gas concentration-sensing equipment directly to the atmospheres within the containers of the perishables. Rather, the systems required means for pumping gaseous atmospheres in the containers through piping to the controller. Such controllers also suffered from exposure to heat in the electrical control cabinet of the container. These systems were vulnerable to failures from any or all of their disadvantages.
All of the prior art systems, regardless of performance, required high capital costs, and imposed substantial penalties on the storage and transportation systems by increasing their weight, their size, and their energy consumption.